Rear Defroster Relay Repair

TCGGrafix · 11-30-2001, 09:29 PM

This is my first post (of many, I'm sure.) I bought my second 911 about a month ago, after selling a 71 911T and figured it was cheaper than therapy for the depression following the sale. (Never again!) This BBS it fantastic! I found it shortly after my latest purchase and have read though about everything that is pertinent to my car. Seems to be a great bunch of people! Anyway...to the subject at hand.

My rear window defroster only works in the second, higher wattage position. The second position will only defrost the top and bottom 3rd of the window. I had my wife pull the switch into the first position while I listened for an audible "click" from the relay in the engine compartment. It would click in the second position but not the first so I assume that the relay is bad. My question is if anyone has attempted to service the internals of the relay. A new one is over $100.00.

Any help is much appreciated!

Early_S_Man · 11:38 AM

Bryan,

I assume you mean this relay, right?

It is an 'electronically-controlled' two-stage relay, which means the two relays are on a circuit board, along with four transistors, on capacitor, one trimpot, and nine resistors.

As shown in the following diagram, the first stage has to operate BEFORE the second stage gets power that can be switched by relay #2 to provide power to the stage #2 grids, so it looks to me like your relay is OK, and that the problem is corrosion on terninal 87, or maybe at the T-21 connector somewhere in the body wiring harness, or maybe one of the two connectors at the heating grid for the middle (critical) section of the heating grid! You may want to do some checking with a digital multimeter at the back window, first, before looking at other areas! Be very, VERY CAREFUL ... with the connectors at the window's heating grids!!! Good luck!

Theoretically, you could replace the "Rear Window Defogger, 911 (1974-89) [Photo]
C-615-115-00 $101.60" ... with two $3.50 simple generic ice cube DIN relays, mounted in $2.09 sockets (#45-287 & #21-193) from the following supplier:

http://www.hosfelt.com/ --> Relays --> 12 Volt --> #45-287

Early_S_Man · 12-01-2001, 04:26 AM

Bryan,

C-O-R-R-E-C-T-I-O-N ...

I disassembled my defogger relay, and it is the same part as yours ... and there is a not-so-minor ERROR in the'82 SC diagram pertaining to the power for the 2nd stage relay to the heating grid! The internal junper is not to pin 87, but instead to pin 30, so the 1st stage relay MAY be bad, or at least not operating at this time! i have edited the drawing and posted it below with the corrrection in red. I would say to go ahead and pull your relay and do some bench testing on it:

1. Check the resistance from terminal 31 to terminal 86 ... mine measured 92 Ohms, and I am sure anything +/- 9 Ohms would be fine.

2. With test lead jumpers, apply +12 Volts to terminal #86 and ground to terminal #31 ... and see if the relay #1 operates.

3. If relay #1 operates check the resistance between terminals #30 and #87 while the relay is operated ... it should be less than 1 Ohm.

Good luck, and let us know what you find!

TCGGrafix · 12-01-2001, 08:49 AM

Warren - If I lived by you I would be over with a 6 pack in hand! Very helpful information! Thanks for taking the time to do it!

I am going to pull the relay this afternoon and check it. I suspect that the coil is bad on the 1st relay or it isn't getting power because of not hearing the click after the first position on the switch is selected. The 2nd relay makes a very noticeable click.

Also, am I correct in assuming the first position only heats the center element? Looking at the schematic that appears to be the case. Also, If I find that I have a short going to relay #1 and decide to do a half arse work-around is it possible to jump the 87 terminal to the 87A terminal to get power to both heating elements without overtaxing the circuit? I know this is the lazy way to do it, but I just want to be able to defog the entire window (for now at least.) One more Q. What is the front window defogger? Is that the side mirrors heating?

Again, thanks for your help!

Doug Zielke · 12-01-2001, 11:18 AM

The very first day with my car I was driving it home on a humid evening. The rear window defog would only work as described above. Found there was nothing more required than a good cleaning of the relay socket and pins. Used CRC Electrical Contact Cleaner. The engine room of the 911 can be a fairly grimy and damp place for poorly sealed electrics.

TCGGrafix · 12-02-2001, 11:36 AM

I tested the relay and everything looks fine - I tested all ground and continuenty of the heating grid and all power supplied into the relay. The only thing I found that was strange was I was only getting 1.5V to the 1st relay at pin #86. I got zero volts with the switch off but would register 1.5V with it on. (?!?!) Is this right? These are 12V relays right?

I also tried to clean the contacts (Which looked fine anyway.) to no avail. I suspect it must be the low voltage reading for the power to the 1st relay at pin #86. Any ideas?

Thanks for your help!

TCGGrafix · 11:46 AM

One more thought - Could I bridge the #86 and #86a connections so the when I put the switch into the second position it powers up both relays (therby supplying power to the entire heating grid.) without screwing anything up. Would the circuit support the load of both relays? Time for rig fest 2001!

Early_S_Man · 12-02-2001, 12:50 PM

Bryan,

Yes, the signal at terminal 86 should be in the 9-11 Volt range for reliable relay operation. Try cleaning the forward 14-pin connector to the body wiring harness with isopropyl alcohol and nylon brush or old toothbrush. You should be careful not to damage the silver-plating on the pins

Yes, as a last resort, you could bridge terminal 86 to 86a to operate the stage #1 relay.

Schulisco · Schulisco's Garage

Hi guys,

this is my very first post here, I'm from Germany and I primarily post in the german forums like elfertreff.de and pff.de. Summer here is in his final phase, we're gonna see autumn coming and therefore the heating in our beloved 911s is getting more and more important. Therefore I dealt a bit with the rear defogger relay 91161511500 because the heating also won't work in my 911.

I googled around quite a while to get more information on it but I couldn't find that satisfying information. I missed detailed information about the exact purpose and function of the 911.615.115.00 relay, which is called in German as a "backwindow heater controller". This name is heavily confusing everybody handling this thing and wondering why the back window won't be heaten any more. I figured out the exact function, and I think this is worthful for you as well that I mention it here as well - later ;-)

If this rear window won't heat any more, first of all, check out all electrical connections of the heatened window and the mirrors (when you have power mirrors in your car, they will be heaten as well on the SCs and Carreras and probably as well the earlier ones, don't know exactly). If this is all okay, and the 'signal' from the switch in the dashboard to #86 and #86a of the silver box is also coming to the relay, any you hear the relays inside clicking, then refer to the contacts of both relays! They will be cooked over the decades, especially that one from stage #2 because it handles a lot of power (600W/14V=>40A!). So mainly check out the full voltage is coming out at #87 and #87a. The resistance at the relay power contacts will rise over the time so you will see a lot of voltage loss there. Replace the two relays with a powerful version which capable to handle 40A or more! More is better;-) Also make sure that the connectors are all clean and free of corrosion!

As promised before - here's the exact purpose of this so called "controller": it handles under voltage supply only!! The first stage always works under any circumstances! There's no magical control in between! But for stage #2! This works as normal meaning it will directly switch the stage #2 of the back window heating as normal as long the voltage is higher than approx. 11,2V!
If the voltage falls under 11V it will cut off the power for stage #2 until the voltage rises over approx. 13,5Volts again! Then it powers stage #2 on again and will keep it as long you manually switch it off on the dash or if the voltage falls again below 11,2Volts!
There's no other control by time or so! So the main purpose is some kind of a 'load shedding relay' in case of starting the engine or similar heavy electrical load or a bad alternator, a bad battery or so...

HTH! Take care!

Regards, Thomas

Knight77911 · Knight77911's Garage

Thank you!

Quote:

Originally Posted by Schulisco

Hi guys,

this is my very first post here, I'm from Germany and I primarily post in the german forums like elfertreff.de and pff.de. Summer here is in his final phase, we're gonna see autumn coming and therefore the heating in our beloved 911s is getting more and more important. Therefore I dealt a bit with the rear defogger relay 91161511500 because the heating also won't work in my 911.

I googled around quite a while to get more information on it but I couldn't find that satisfying information. I missed detailed information about the exact purpose and function of the 911.615.115.00 relay, which is called in German as a "backwindow heater controller". This name is heavily confusing everybody handling this thing and wondering why the back window won't be heaten any more. I figured out the exact function, and I think this is worthful for you as well that I mention it here as well - later ;-)

If this rear window won't heat any more, first of all, check out all electrical connections of the heatened window and the mirrors (when you have power mirrors in your car, they will be heaten as well on the SCs and Carreras and probably as well the earlier ones, don't know exactly). If this is all okay, and the 'signal' from the switch in the dashboard to #86 and #86a of the silver box is also coming to the relay, any you hear the relays inside clicking, then refer to the contacts of both relays! They will be cooked over the decades, especially that one from stage #2 because it handles a lot of power (600W/14V=>40A!). So mainly check out the full voltage is coming out at #87 and #87a. The resistance at the relay power contacts will rise over the time so you will see a lot of voltage loss there. Replace the two relays with a powerful version which capable to handle 40A or more! More is better;-) Also make sure that the connectors are all clean and free of corrosion!

As promised before - here's the exact purpose of this so called "controller": it handles under voltage supply only!! The first stage always works under any circumstances! There's no magical control in between! But for stage #2! This works as normal meaning it will directly switch the stage #2 of the back window heating as normal as long the voltage is higher than approx. 11,2V!
If the voltage falls under 11V it will cut off the power for stage #2 until the voltage rises over approx. 13,5Volts again! Then it powers stage #2 on again and will keep it as long you manually switch it off on the dash or if the voltage falls again below 11,2Volts!
There's no other control by time or so! So the main purpose is some kind of a 'load shedding relay' in case of starting the engine or similar heavy electrical load or a bad alternator, a bad battery or so...

HTH! Take care!

Regards, Thomas